The present invention relates to a horizontal sync signal mode discriminator for display monitors, in particular, a mode discriminator which can discriminate a particular horizontal sync frequency mode by means of the polarity of vertical synchronizing signals.
According to the conventional art shown in FIG. 3 (A), the horizontal synchronizing signal is applied through a first-stage stable multivibrator 31 to a second stable multivibrator 32, whose outputs are used for maintaining horizontal sync voltage, for stabilizing vertical synchronization and for controlling a double mode switch.
According to the conventional art employing a waveform in FIG. 3 (B), upon applying horizontal sync frequency of mode 1 the-first stage stable multivibrator 31 detects the rising edge of the input signals of period TW1 and delays a change in output for a preset time TW3 as shown in FIG. 3 (C). The output waveform is as in FIG. 3 (C).
The second stage stable multivibrator 32 detects the rising edge of the waveform shown in FIG. 3 (C) to delay a change in output for a time TW4 as shown in FIG. 3 (B). However, since the time TW3 is smaller than TW4, the output waveform of the second stage stable multivibrator 32 remains at a high level voltage as shown in the left side of FIG. 3 (D) and is supplied to the horizontal hold switch circuit and the stabilizing vertical synchronization circuit to cause a desired operation, and is also supplied to the double mode switch so as to compensate for the effects of horizontal frequency variations. Upon applying a horizontal sync frequency of mode 2, multivibrator 31 detects the rising edge of signals of period TW2 which is smaller than delay time TW3. Therefore, the output of multivibrator 31 remains high as shown in the right side of FIG. 3 (C). As such, the output of multivibrator 32 remains low as shown for mode 2 in FIG. 3 (D). However, in practice, the error of capacitors C1 and C2 and the difference of pulse width between times TW1 and TW2 are too small for the main correction resistor VR1 to compensate for, thus resulting in the disadvantage of lowering the productivity and reliability of the product.